Magnetic flux meter



Oct.l 28, 1941. G. s. SMITH MAGNETIQFLUX METER 2 sheets-sheet 1 FiledJuly 24, 1939 1 2 FL al Mfrf/P MM am@ erss4aa1oo Oct 28, 1941- G. s.SMITH 2,260,589

MAGNETIC FLUXMETR Filed July 2.4, 1939 2 sheets-sheet 2 FL UA MITEIPP640/IVG 'III'.IAYIIIIIIIIIIIIA INVENTO or transient ilelds. A

Patented Oct. 28,1941

AUNITI-:D STATES vPATENT orrlcl:I

' 2,260,589 j GTTL Application July 24, 1939, Serial No. 286,094:

4 Claims.

My present invention relates broadly to electrical metering apparatus,and more particularly to a magnetic flux meter and a novel method ofmeasuring magnetic flux intensity. In my present invention it is thecombination of bismuth wire in'v a Wheatstone bridge with certaincurrent measuring devices which constitutes as a whole the magnetic uxmeasuring instrument.

It has long been known that the electrical resistance of bismuth isaffected by the presence of magnetic flux by means of bismuth, thisprocedure required the very careful measurement of the resistance oieredby the bismuth wire in the field, as well as that in zero field. AThepercentage` change in resistance thus obtained is referred forinterpretation to a calibration curve. evident, this method iscumbersome for general use, and requires care and skill for accuracy.This is due primarily to a small change in temperature which maybccurbetween measurements and will often result in a considerable yerrorinthe result. It is, therefore, evident that this method is not welladapted to cont uous measurement.

Heretofore the usual procedure was to measure the resistance of thebismuth by means of a Wheatstone bridge. Howeven in such a setu'p ytheWheatstone bridge is merely incidental to the measurement of the bismuthresistance. In my invention I do not measure the exact resistance of thebismuth wire nor the percentage change of resistance therein, but ratherthe unbalanced current in the bridge due to unbalanced rer. sistancesresulting from the presence oi magnetic iiux. Y

i ne oi the main objects of this invention is to provide a meter formeasuring magnetic iiux intensities in alternating current, -directcurrent Another object of; this invention is to provide 'a meter whereinthe temperature resistance coefilcients of the various elementsaresubstantially.

canceled out.

Afurther object l vide a meter in whichthe oflthis invention isprothermo-electric eect vof dissimilar metals are essentiallyneutralized.

A still further object of the invention is to provide a means fordetermining the direction of the magnetic vector beingmeasuredn Anotherfeature of this invention is to provide l0 a magneticleld. In the priorart of measuring diagram of the entire instrument.

Figure 2 is a.` sample calibration chart for the flux meter.

Figure 3 shows the bridge or exploring probe and the terminal blockassembly.

Figure 4 shows an end view of the Jack.

Figure 5 shows an enlarged-section through line E--5 of Figure 3. l

Figure 6 is a graph showing the directional eect of the magnetic iiuxwhen passing through the exploring coils.

Referring to the drawings, throughout which Figure 1 is the schematiccircuit diagram of the magnetic iiux meter. The Wheatstone bridgeportion of the circuit is composed of the four resistances shown at 2.I, 6, 8, and a fth resistor, which is adjustable, shown at I0. The twodiametrically opposite resistors shown at 2 and 8 are the bismuthspirals, the electrical resistance of which is a function of themagnetic flux pass ing through them. 'I'he other two arms indicated at 4are resistances which are practically non sensitive to the magneticileld.

To balance out anyalternating voltage still generated in the bridgecoils. an alternating cur 'rent compensating coil of a yfew turns 'ofcopper 40. wire is shown at i6 and connected across potentiflux beingmeasured it may be. calibrated to read l Y the magnetic ilux intensity.

Thermocouple 2i is built into 4the bridge and L its terminals areVbrought out at 28 for connecting like reference characters indicatelike parts,

nals at 30 arefor connecting .to an oscillograph- 5 when transientiields 'are to be measured.

The calibration graph of Figure 2 shows three families of characteristiccurves for this bridge. All curves were taken at a constant temperature.

The'full line indicates 4the calibration for direct n,

current iiux, while the dashed line indicates the alternating currentcalibration. The three families of curves A, B, and C, result from threevalues of currents used in'the bridge, namely,

'three milliamperes in the case of A, 10 millila ab=temperaturecoefficient of bismuth at the base amperes for B, and 25 milliamperesfor C. The dashed curve indicates average values of alternating currentux. i

Figure 3 shows in more detail the arrangement of all elements of thebridge shown exter 2n nally to the 4dashed enclosure in vFigure 1. Thesensitive portion of the exploring probe is shown at 32 and contains thefour arms of the bridge plus the compensating coil and the thermocoupleelement. These are connected by suitable foil 2;,

leads to the Jack shown at341' The plug 36 connects the exploring probeto the other meter elements through a suitable cable 38.

Figure 4 shows the detailsof the jack 36. f Figure 5 shows an enlargedcross-section along 3i line 5-5 of Figure 3. The leads from the a1-temating current compensating coil is shown at I1. The mica support andcover strips'are indicated at 42, while the copper foil lead strips areshown at 44. The lead strips are separated by 35 sheets `of paper at 48.Curves A and B in Figure 6 show respectively the effect of the magneticux being parallel to the plane of the exploring coils and perpendicularto the plane of the same coils. 4u

Basically, the operation of this magnetic Aiiux meter depends upon thechange in the -electrical resistance bismuth exhibits when placed in amagnetic eld. vIn addition to the usual elements found'in a Wheatstonebridge, several novel re- 45 nements have been incorporated in thisinstrument to eliminate certain undesirable errors. Two bismuth'resistors as shown at 2 and 8 in Figure 1- are placed in thediametrically opposite armsof a Wheatstone bridge, and in zero mag- 50netic iield, the bridge isaccurately balanced by -using an ordinaryadjustable resistance in`one or more of the remaining arms such as shownat j I0 in Figure 1. When placed in a magnetic eld,

the bridge is unbalanced-by an amount depend- 55,

ing upon the eiectof that fleld upon the bismuth resistance. Thus thecurrent, or voltagll due to this unbalance can be calibrated directly interms .of the magnetic eld density., This unl balanced current, orvoltage, is indicated by me` o ter 22 in Figure .1'. vThe meter used inthev particular instrument'was a model 600 Weston, 0-500 l microammeter,with a resistance of 10 ohms.

Td avoid changes in the zero setting of the meter 22, due to temperaturevariation, all the 65 resistors 'in the bridge are adjusted to. have thesame temperaturewesistancechange. To accomplish-this, a-metal of highertemperature coefficient of resistance than bismuth was used, and onlyenough so that its resistance change would 70.

always be equal to the corresponding resistance The remainder fsthe sresistance or'some similar matebe made equal to aero forthe usualworking temperature, or. for any temperature range over which thetemperature coeiiicient of the metals R0=Rtabmq Where Ro=resistance ofthe balancing resistor at the assumed hase temperature Ra=resistance ofthe bismuth resistor at the assumed base temperature ao=temperaturecoeicient of the balancing re sistor material at the base temperaturetemperature l In some of the first bridges built, nickel wire was usedfor the resistance arms 4 and 6 in Figure 1. However, it was found thatunder certain conditions which will be discussed later a slight negativereading was obtained. This disadvantage was eliminated by thesubstitution of either lead or tin wire for the resistance arms 4 and 6in Figure 1.

All four of the resistance arms'2, 4, 6,' and 8 in Figure 1 are wound inat non-inductive spiralsas shown in Figure 7. 'The bismuth spirals 2 and8 were placed between the other resistance spirals 4 and 6. A verysudden change in temperature may result in uneven heating of the variousspirals, and thus a temporary shift of the zero. This could be greatlyreduced by interwinding the probe as shown at 32 in Figure 3. The mainbody of t exploring probe is made up of a central supp rting'` strip ofany suitable insulating mate' rial, copper foil conducting strips,separated by a thin layer of insulation, and nally a probe pro-vtective. strip on the outside. The exploring probe is thus built up oneach side from the central supporting'strip. The bridge design shown inFigure 3 and described above was necessary to avoid undesirablethermoelectric effects and also to balance out any induced voltage inthe foil leads when used in alternating current, or transient fields.since bismuth has a rather nigh thermo-electric eiect, when joined tothe majority of metals, a considerable amount of error might result itthe junction of the various ends of the spirals were not'kept at thevsametem perature.V The copper foil and insulating strip constructionaccomplishes this purpose very satisfactorily. All junctions were madeas near each other as possible so that very little diil'erence in theirtemperatures could`exist. Furthermore, the

thinness of the exploring probe shown in Figure 3 magnetic air gaps.

A third problem presents itself when the temperature of the bridgevaries anyc'onsiderable amount from that at whichthe bridge is -calilbrated. When a considerable diil'erence in temperature is involved somecorrection must be made to compensate for the change of sensitivity.

if any great accuracy is desired. When in aconstant magnetic iieldbismuth has the characteristic oi an increasing change of resistance asthe temperature decreases. in order to determine the variation in thelbridge temperature, a small thermo-couple may. be built into the bridgewith one junction pilzled between turns, as far into the blsmuthsp aspossible. The other Jime- :incassa4 v 3 tion may be placed in the metercase where a thermometer could be in contact with it. 'I'he thermocouplemay then be calibrated for the difference in temperature between thesejunctions. Such a thermocouple is shown at 2l, its cold junction at 21,and terminals -for the galvanometer at 28, in the schematictdiagram ofFigure 1. Although the galvanometer for the thermocouple 26 is`indicated as being connected externally to the case of the ilux meter,it could just as well be included as a`n integral part of the meter. v

Instead of taking corrections from a chart or graph or by means of acorrection formula, some automatic or manually operated device might bedevised to alter the sensitivity of meter 22, or

the resistance of some other portion of the circuit, or introduce somecompensating voltage to correct the error in the proper sense. l

For certain types of work where space is available for enclosing theprobe in a special temperature controlling compartment, the probe may bemaintained at some desired constant temperature for obtaining accuracyin readings or for operating the bridge at some more optimum'xtemperature or both.

It has been found impractical to design the bridge spirals to beabsolutely non-inductive. Even with great care there may still be someunbalanced alternating voltage induced in one or more arms of thebridge. I-t is therefore much more simple to balance out any alternatingvoltage generated in the bridge coils by means of an alternating voltagecompensating coil of a few turns of copper wire wound around the outsideof one set of the bridge coils. Such an alternating compensating coil isshown at I6 in Figure 1.

The alternating voltage developed by this coil when in an alternatingfield is applied across the 'potentiometer I2, one side of which isconnected to the center terminal of the single pole double throw switch3|. By means of the sliding contact I3 on potentiometer I2, the requiredamount of voltage may be selected to cancel out that which is induced inthe spirals of the bridge. It is of course necessary that theinstantaneous polarity across potentiometer I2 have such a relationshipas to oppose the voltage induced in the bridge itself. To make theadjustment of cananother may be designed with a very much larger 'varea, but sensitive to much lower field densities. In this way, theutility of the meter as a whole is very greatly enhanced.

The alternating current calibration curves shown in dashed line inFigure 2 were obtained from tests for frequencies varying from 12 to 540cycles per second. Below 12 cycles, the instrument pointers vibrated toomuch for a reading. No apparatus was availableto go above 540 cycles,and theoretically this magnetic flux meter should give satisfactoryresults for frequencies several times the limits set by the apparatus.All values taken over this range of frequencies were seldom over two percent. above or below the average value shown in the dashed line ofFigure 2. There seems to be no tendency whatever for the values taken athigher frequencies to be either higher or lower than the other.

1inI addition Vto metering direct current flux, alternating currentflux, and transient flux, this meter will also indicate the directionofthe magnetic iiux vector being measured. In Figure 6 is l shown acalibration curve wherein curve A shows ceiling any induced alternatingvoltage anoscillograph may be connected to the terminals shown at 30 inFigure 1. Switch 3| is thrown in the proper position to connect theoscillograph in the circuit. The exploring coils are then placed in astrong alternating magnetic` ld andthe meter batteryv circuit isleftopen. The sliding contact I3 on potentiometer I2 is then adjusted forzero response in the oscillograph connected to the terminals 30. Aftervthis adjustment is once made no further alteration is necessary. Whenthe instrument is used for direct current, this coli has no function andisvirtually outside the working circuit.

The method of construction shown in Figure 3,' wherein the exploringprobe elements areconnected tothe meter box by means of a jack, plug,

' and cable wasresorted to in order that several exploring probes ofdifferent characteristics could be used with the same meter box and itscomponent instruments. This method ailowsthe bridge coils to be easilydetached for checking or repairs but is of most value in making adjustment during construction. For example, if the meter is supplied ywithmore than one bridge, the elements of one may be very small in diameterfor measuring fields of very small area. while 'l5 meter readings whenthe magnetic flux is parallel to the plane of the coils or spirals ofthe exploring probe. Curve B was obtained by having the magnetic iiuxperpendicular to the plane of the coils or spirals in the exploringprobe. It is therefore obvious that to find the position of the magneticflux vector it is only necessary to' change the position of theexploring coils in the magnetic field until a maximum reading isobtained on the meter, in w V ch position the magnetic flux vector isperpendicular to the exploring coil plane. It has been found that ifnickel wire is used for the balancing spiral a negative reading, will beobtained in meter v22 when the exploring coils are placed parallel .tothe magnetic flux lines, if the flux densitiesare relatively low. ofnickel for the balancing spirals, no negative readings are obtained andhence this disadvantage is eliminated.

In the construction of several bridges and-exploring probes, it wasfound that ordinary solders have such high melting points that thebismuth wire would be melted before a joint could be made. By using aforty per cent. lead, sixty per.

- tioned asthe electrically resistive magneto sensitive material for thediametricallyI opposite arms of the Wheatstone bridge, any othermaterial having similar characteristics could, as well. be

used in a magnetic flux meter i.of this -type and" still come under thescope of this invention.

Likewisathemethod of compensating, for the voltage induced in thebridge, by means of the compensating coil is merely one` of severalmethods which could be employed-'to accomplish the Any method of'compensatingv for saidy induced voltage would be weil within thezin-.tentof this invention.

same purpose.

Since the sensitivity of the magnetic iiux meter By using lead ortinwire instead changes with a change of temperature, it is obvious thatcorrections may be made by changing -the sensitivity of meter 22 in anopposite sense vand the use of such a method of correction or the use'of other compensating devices in the form of altering the resistance orby altering the voltage in the proper portion of the circuit would stillbe within the concept of this invention.

It has also been found to' be advantageous to surround the bridgeelements with some suitable material for maintaining them at someconstant or predetermined temperature, or for operating the bridge atamore optimum temperature.,

Although specific materials and specific qualities of such materialshave been mentioned for the purpose of maintaining the balance of theWheatstone' bridge in spite of temperature sistance member, a source ofelectric energy connected serially through resistive means and ametering device to said bridge, a compensating coil disposed in such amanner as to'be inductively linked by the magnetic ux under measurement,a metering circuit comprising a current indicating device connectedserially through a resistance and said compensating coil in parallel,and said metering circuit connected to the nom- Y,

inally neutral points on the bridge. n

3. A magnetic ux meter comprising a fourarm resistance bridge having twodiametrically Opposite arms composed'mainly of bismuth. one arm of saidbridge including an adjustable resistance member, `a source of electricenergy connected serially through resistive means and a mechanges, othermaterials and other lqualities of 1 stone bridge having two of its armscomposed mainly of bismuth, adjusting means for obtaining an electricbalance in the bridge, a source of4 energy for electrical excitation ofsaid bridge.

means for adjusting the ow of energy to the bridge from said source,means for determining the amount of excitation being supplied to saidbridge, means for `indicating the unbalance energ? in the bridge, and acompensating coil absorbingenergy from the magnetic eld undermeasurement and applying a portionto oppose any-unbalanced energyinduced in the bridge by the magnetic-eld being measured.

2. A magnetic flux meter comprising a fourl arm resistance bridge havingtwo diametrically opposite arms composed Jmainly of bismuth,- oneI armof said bridge including an ladjustable retering device to said bridge,a compensating coil disposed in such a manner as to be inductivelylinked by the magnetic iiux under measurement. a metering circuitcomprising a current indicating device connected serially through aresistance' and said compensating coil in parallel, said meteringcircuit connected to the nominally electrically neutral points of thebridge, and adjacent to the said bridge armsV thermo-electric meansconductively connected to a current indicating device.v

4. A magnetic ilux measuring device compris-5v ing a four-arm resistancebridge, two diametrically opposite arms of said bridgecontainingelectrically-resistive-magneto-sensitive elements, one arm ofsaid bridge including an adjustably ,resistive element, a source ofelectric energy conductively coupled serially through a meteringdeviceand adjustably resistive means to electrically and diametricallyopposite points on the bridge, a metering circuit comprising a metering,device conductively coupled serially through adjustably resistive meansto the neutral points `of said bridge and a compensating coil disposedin the immediate region of the electricallyeresistivevmagneto-sensitiveelements of the aforementioned bridge and conductively coupled inparallel with the adjustably resistive `means included in theaforementioned metering circuit.

GEORGE s. sMrrHL

